The present application is directed to techniques for testing a water-cooled generator for leaks, and more particularly to improvements in the process for drying water channels in the generator""s stator to prepare the stator for pressure-decay testing and vacuum-decay testing.
FIG. 1A schematically illustrates a stator 10 of a water-cooled generator. The stator 10 comprises of stator bars 14, as shown in FIG. 1B, which are formed by uniting a number of individual conductors 16 in a meandering pattern. The meandering pattern (transposition) helps minimizing electrical losses. A cross-sectional view of one of the conductors 16 is shown in FIG. 1C. As will be seen, the conductor 16 has a channel 18 for passage of water to cool the stator. Although not shown, the stator bars 14 are covered with electrical insulation.
A metal fixture called a water box (not illustrated) is connected to each end of every stator bar 14. Further metal fixtures (not illustrated) are then used to hydraulically connect adjacent water boxes and to also electrically connect the stator bars 14 so as to form stator windings. These further metal fixtures are hydraulically connected by electrically insulating fixtures to an inlet header 20 at one end of the stator 10 and to an outlet header 22 at the other end. Thus, by way of the various fixtures, one end of each stator bar 14 is in hydraulic communication with the inlet header 20, and the other end of the stator bar 14 is in hydraulic communication with the outlet header 22. FIG. 1A shows the stator bars 14 in one stator winding loop of the stator 10.
Coupling members 24 and 26 are provided outside the stator 10 to provide fluid communication with the inlet header 20. Similarly, coupling members 28 and 30 are provided outside the stator 10 to provide fluid communication with the outlet header 22. The coupling members 29-30 are accessible from outside the generator itself.
It will be apparent that a number of components are connected together to form the stator 10. Many of these connections are brazed connections, which are susceptible to failure. Water leaks may also form in regions other than the connections. Although the water used for cooling the stator bars 14 is pure, and thus has a very low conductivity, any water that leaks out of joints of the fixtures connected to a stator bar 14 may soak its insulation and thereby degrade the insulation""s ability to withstand high voltage. In particularly severe cases, a short circuit due to water leakage may cause catastrophic failure of the stator 10.
Due to the risk of generator failure, or the risk of an unscheduled shut-down for less severe leakage, it has become common practice in the power generation industry to periodically test water-cooled generators for leaks. The General Electric Company, a major manufacturer of generators, has published information about leakage testing in Technical Information Letter Number 1098, one version of which was published on Jan. 24, 1995 and was updated by Alan M. Iversen in November of 1996. Further information about testing is provided in a paper by Bruce Faulk et al., entitled xe2x80x9cDiagnosing and Repairing Water Leaks in Stator Windings,xe2x80x9d that was presented at an EPRI (Electric Power Research Institute) conference in 1995.
The periodic testing typically begins by draining the stator and then drying it. After the stator has been thoroughly dried, pressure decay and vacuum decay tests are conducted to confirm that the stator is able to hold pressure and vacuum. If the pressure within the stator falls too rapidly after the stator has been pressurized with compressed air, or if the pressure rises too rapidly after air has been evacuated, then a leak that requires further attention has been detected. The generator is then opened so that further testing can be conducted to identify the site of the leak so that it can be repaired.
Equipment for drying a stator and for conducting the pressure and vacuum decay testing may be collected together on a chassis or housing to provide an arrangement called a test skid. A conventional test skid typically includes an air tank or air receiver for holding compressed air. A compressor is not needed, since most power plants have piping systems for delivering compressed xe2x80x9cinstrument airxe2x80x9d (cleaned and dried air for instrumentation) and compressed xe2x80x9cservice airxe2x80x9d (utility compressed air for pneumatic tools and other applications where instrument air is not necessary). A conventional test skid also includes a vacuum pump. An arrangement of conduits, valves, and sensors for conducting pressure and vacuum drying and for performing the pressure and vacuum decay tests themselves is also present. Auxillary equipment, such as conduits for connecting the skid to the generator, may also be housed on the skid.
The drying procedure using a conventional skid is typically conducted in two stagesxe2x80x94a pressure drying stage and a vacuum drying stage. After the stator has been drained, the air receiver is charged with air to a predetermined pressure; the pressurized air is introduced to the stator via one of the upper coupling members 24 or 28; and then the pressurized air is discharged from the stator via a diagonally disposed bottom coupling member 26 or 30. After the pressure falls to a predetermined value, the air receiver is again charged with pressurized air, and this pressurized air is discharged through the stator. A number of such pressure drying cycles are conducted. Next, the stator is closed and air is evacuated. Water that remains trapped in tiny nooks or crevices after completion of the pressure drying stage evaporates into the vacuum and is removed. When the stator is finally dry enough, as indicated by vacuum and dewpoint sensors, it is charged with compressed air to a predetermined pressure, and the pressure is measured periodically during a monitoring interval to determine whether the pressure decays or falls at an acceptably slow rate. During the vacuum decay test, the stator is evacuated and measurements are made during a monitoring interval to determine whether the vacuum decays (in this case, meaning that the sub-atmospheric pressure rises) at an acceptably slow rate.
The drying procedure in preparation for the pressure and vacuum decay testing can be conducted fairly expeditiously if it is begun while the generator remains hot, soon after it has been taken off line. If there is a delay, however, the drying procedure may take a week or more, particularly if the generator is housed in an unheated building. One reason for this may be that ice crystals form due to cooling because of rapid evaporation of water droplets during the vacuum drying stage. The ice crystals sublimate slowly into the vacuum, and moreover may clog crevices which contain residual water.
More than a year before the present application was filed, the inventor observed a test skid that was being operated by employees by a company then named MDA, or Mechanical Dynamic Analysis. The skid included a small heater which slightly raised the temperature of compressed air entering the air receiver. The purpose of this may have been to avoid water condensation inside the air receiver.
An object of the present invention is to provide an improved method for testing water-cooled generators and a test skid for use in the method.
Another object is to provide a method to accelerate drying-out of a stator before pressure and/or vacuum decay testing is conducted.
Another object is to provide a test skid in which the same dewpoint sensor may be used during both pressure decay and vacuum decay testing.
These and other objects which will become apparent in the ensuing detailed description can be attained by providing a method for testing a water-cooled generator having a stator with water channels, which method includes the steps of drying the water channels and then conducting at least one of a pressure decay test and a vacuum decay test. The step of drying the water channels includes receiving compressed air into an air receiver, conveying compressed air from the air receiver along a flow path from the air receiver to the water channels of the stator, and heating the compressed air as it moves along the flow path from the air receiver to the water channels.
When hot air is injected into the stator during the pressure drying stage, the hot air loses heat to the stator. The heat transferred during a number of cycles of pressure drying raises the temperature of the stator. As the stator temperatures rises, the loss of heat by the incoming compressed air to the stator is reduced, so that the air temperature also rises. Psychrometric charts show that the water retention of air depends in a complex way on the temperature of the air, but in general it can be said that water retention increases rapidly with increasing temperature. For example, the amount of water that can be retained as vapor in one pound of air at 80xc2x0 F. is about four times as great as the amount of water that can be retained as vapor in one pound of air at 40xc2x0 F.
The increased moisture-carrying ability of heated air accelerates removal of water from the stator during the pressure drying stage. It also leaves the stator warm at the start of the vacuum drying stage. This means that ice crystals are less likely to form in the first place, since the temperature of the residual water in crevices and so forth is warmer to begin with, and also that stator heat is available to prevent excessive cooling during the vacuum drying.